Designers of power drives for PWM Motor Controls and switching power supplies typically face the problem of driving a high-side MOSFET transistor in a high voltage power stage. A common technique used in high-side drive circuits is to generate an isolated auxiliary supply voltage. This supply voltage provides power to a conventional gate-drive circuit. The average auxiliary power consumed is generally well below 1 watt, and varies with switching frequency, FET size and number of paralleled FETs used to configure one switch.
Typically, an opto-coupler is used to translate the switch activation command from a ground referenced, or low-side, control circuit up to the high-side driver. This technique, however, creates reliability problems, including low common mode transient immunity, and performance degradation over time and temperature. In addition, high voltage MOSFET circuit slew rates can easily exceed 20 kV/.mu.s, causing opto-coupler self-turn-on or turn-off. The opto-coupler's AC common mode rejection must be carefully evaluated, as this characteristic is typically influenced by common mode voltage as well as dV/dt. Power up and power down sequences also present potential failure without undervoltage lockout circuitry. See Unitrode Application Note U-127, entitled Unique Chip Pair Simplifies Isolated High Side Switch Drive.
One shortcoming of many prior art gate-drive circuits is that a separate DC power supply is required. Further shortcomings are that many prior art circuits are too expensive, have too many parts, are limited to a 50% duty cycle and/or perform poorly. In addition, most gate-drive ICs do not provide negative turn-off voltage, which would provide noise immunity if available.